I have a big confusion since the very beginning I started dealing with gridded rainfall data.
Suppose I am using 0.25 gridded rainfall data to generate a rainfall time series for one of the districts of India and approximately 6 grides as per latitude and longitude are lying within that district. So do I have to take the average rainfall of those 6 grids or sum of the rainfall of those 6 grids for considering it to be the data for that particular district?
Is this the right approach?
So far as I think the gridded data which one download at any scale is the interpolation and EXTRAPOLATION based on the historical events over a particular spatial domain. So for taking rainfall at grid level it should be the average and not the sum of n number of location in the district. If you add it that will give the total rainfall amount in all the 6 grids. Average will tell us the single figure as a representative of a land covered by all the six grids all together.
I agree with Dr. Singh, averaging. However, this depends on district boundaries agreeing with grid boundaries. As suggested by Dr. Singh, each grid point represents that area between or among the adjacent grid point(s). So if you have a district or watershed boundary, that grid point that is fully contained within the boundary has more weight than a grid point that lies near the boundary. And in some instances, you might have a nearby grid point just outside the boundary, that has some area of influence. In the old days, we had to graph this all out on paper, and use a dot grid or planimeter to determine the area weights about each rainfall station or grid point in reference to the hydrologic or other boundary of interest. Most hydrology books used to explain how to do this. Today GIS or perhaps other spatial software can simplify. So the averaging done is weighted by the area within the boundary. So if the grid point area of influence covers 10% of the analysis boundary, that is its weight. Using this approach, you add up all the area weighted areas. If you really get interested in the subject, krieging is probably the tool they use to set the grid point values from available rainfall stations.
William F. Hansen . Som Pal Singh Thank you so much, Sir, for explaining the things so clearly.
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